1. Technical Field
The present disclosure relates to a microelectromechanical device having an oscillating mass and a forcing stage and a method for controlling a microelectromechanical device.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has increasingly spread in various technological sectors and has yielded encouraging results especially in providing inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS systems of this type are usually based upon microelectromechanical structures comprising at least one mass connected to a fixed body (stator) by springs and movable with respect to the stator according to one or more degrees of freedom. The movable mass and the stator are capacitively coupled through a plurality of respective comb-fingered and mutually facing electrodes so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors. Thus, by sensing capacitance, it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the force applied. Instead, by providing appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. In addition, for providing electromechanical oscillators the frequency response of MEMS inertial structures is exploited, which is typically of a second-order low-pass type with one resonance frequency.
MEMS gyroscopes have a more complex electromechanical structure, which comprises two masses that are movable with respect to the stator and are coupled to one another so as to have a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to a driving sub-system and is kept in oscillation at the resonance frequency. The other mass is drawn in the (translational or rotational) oscillatory motion and, in the event of rotation of the microstructure with respect to a gyroscopic sensing axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the driven mass, which is capacitively coupled to the fixed body through electrodes, as likewise the driving mass, operates as an accelerometer, which enables detection of the Coriolis force and acceleration and hence makes it possible to trace back to the angular velocity.
In gyroscopes, as likewise in other devices, the movable mass or the system of movable masses is maintained in oscillation at a controlled frequency. This may be accomplished through a driving device coupled to the micromechanical structure so as to form a resonant microelectromechanical loop which vibrates with controlled frequency and amplitude. Clearly, upon turning-on of the device (power-on) or at exit from low-consumption configurations (power-down) a start-up transient occurs before the movable mass or the system of movable masses reaches a stable condition of oscillation.
In the start-up transient, the oscillatory motion is forced through start-up components, which supply a fixed amount of energy, normally by applying one or more sequences of pulses of programmed duration to the movable mass. Once the transient is exhausted, the start-up components are de-activated, and the oscillation is maintained by the microelectromechanical loop that guarantee normal operation.
Sequences of pulses may be generated during start-up transients by a local oscillator embedded in an ASIC (“Application Specific Integrated Circuit”) chip coupled to the micromechanical structure. The overall number of pulses (i.e. the maximum duration of the forcing sequence of pulses) is determined from the residual difference between the oscillation frequency of the oscillator and the microelectromechanical loop. The residual frequency difference, in fact, causes a phase lag at each oscillation cycle between the oscillator output and the natural resonance frequency of the microelectromechanical loop. The overall phase delay therefore increases in time as the number of pulses increases and may lead to a condition in which energy provided by the local oscillator tends to counter rather than favoring oscillation of the microelectromechanical loop. In particular, the overall phase delay cannot exceed π/2 for an efficient forcing.